Materials which have traditionally been used in microelectronic devices due to their electrical properties have recently been utilized to form mechanical structures. Techniques for micromachining these materials to produce miniature three-dimensional mechanical structures such as cantilever beams, trestles, pedestals and other similar structures have been developed. Hence, it is now possible to fabricate micromechanical structures on silicon chips which respond to inertial and other forces.
These micromechanical structures have been used as transducing elements to "sense" vibration and acceleration. In these devices, a micromechanical structure, such as a cantilever beam, is formed such that it responds to an applied force. For example, a cantilever beam can be formed which deflects in response to a force applied normal to its principal surface. The direction and magnitude of a force applied to the beam is proportional to the beam deflection. Thus, micromechanical structures can be fabricated which are excellent transducing elements for vibration sensors and accelerometers. Micromechanical transducing elements can be rapidly and reliably mass-produced using batch fabrication techniques adapted from microelectronics technology. The quality of the cantilever beam or other transducing element is critical to the performance of the sensor.
In order to provide greater packing density and to reduce parasitic capacitances which may interfere with signal transmission, many vibration sensors and accelerometers are designed such that the micromechanical transducing element and the detection circuitry are provided on a signal chip. Since the magnitude of deflection of the micromechanical structure is very small, it is essential that parasitic capacitances be kept to an absolute minimum so that they do not overwhelm the signal generated by the transducing element. By integrating electrical and mechanical components in a single silicon body to produce a "smart sensor", parasitic capacitances can be substantially reduced.
In the past, solid-state accelerometers and vibration sensors were based primarily on the piezo-resistive effect. Piezo-resistive sensors relate a stress-induced change in resistivity of a diffused resistor to the magnitude of the stress-producing force. Other solid-state accelerometers utilize changes in the electrical characteristics of a pn junction when stressed. In general, however, piezo-effect devices are only accurate when measuring extremely large stresses. Moreover, when these devices are highly stressed, the materials of which they are formed often fracture under the strain. Other known accelerometers include cantilever beams which function in the nature of electromechanical switches. The beams deflect in response to accelerations, but the beam movement is used only to close or open an electrical contact, much like a mechanical switch. Some of these mechanical, switch-like devices have an array of cantilever beams. Each beam is weighted or "loaded" to respond to an incrementally greater acceleration by virtue of a greater inertial mass. The devices are capable of measuring a range of acceleration thresholds.
Sensors have also been devised which include a cantilever beam or other transducing element which is integrated with metal oxide semiconductor (MOS) detection circuitry in a single chip. In these smart sensor, accelerations of the chip induce motions in the beam that produce capacitance variations which drive the detection circuitry. The changes in capacitance are extremely small, and the signal so produced is thus susceptible to being overwhelmed by parasitic capacitances. As mentioned above, parasitic capacitances are most easily reduced by placing the detection circuitry "on-board" with the micromechanical transducing element.
Conventional techniques for fabricating smart sensors having one or more micromechanical transducing elements, do not provide a convenient procedure for fabricating both the detection circuitry and the micromechanical structure. One acute problem with prior art techniques is that conventional chemical etchants used to undercut micromechanical structures such as cantilever beams and the like etch the preformed detection circuitry along with the micromechanical structure. Hence, the use of prior art chemical etchants such hydrofluoric acid and ethylenediamine pyrocatechol require that the detection circuitry be protected by complicated masking techniques during formation of the micromechanical structures. It would therefore be desirable to provide a process for fabricating three-dimensional structures such as cantilever beams and trestles which affords a high degree of selective etch control and which does not require complicated masking procedures to protect the detection circuitry. In United States patent application entitled "Method for Patterning Silicon Dioxide With High Resolution in Three Dimensions", Serial No. 836,900, filed Mar. 6, 1986, which is assigned to the assignee of the present invention, a method is disclosed in which an aqueous ammoniacal hydrogen peroxide solution is used to preferentially etch ion-damaged silicon dioxide. We have now discovered a method for producing micromechanical structures which utilizes an ammoniacal hydrogen peroxide solution as an etchant. We have also discovered three-dimensional microstructures for use as transducing elements in vibration sensors. We have further discovered a high-sensitivity integrated vibration sensor which uses standard MOS circuit components to measure extremely small motions of microstructures.